Dual-Echo EPI sequence for integrated distortion correction in 3D time-resolved hyperpolarized 13 C MRI.

PURPOSE: To provide built-in off-resonance correction in time-resolved, volumetric hyperpolarized 13 C metabolic imaging by implementing a novel dual-echo 3D echo-planar imaging (EPI) sequence and reconstruction.

METHODS: A spectral-spatial pulse for single-resonance excitation followed by a dual-echo 3D EPI readout was implemented to provide 64 × 8 × 6 cm3 coverage at 5 × 5 × 5 mm3 nominal resolution. Multiple sources of EPI distortions were encoded using a multi-echo 1 H EPI reference scan. Phase maps computed from the reference scans were combined with a bulk 13 C frequency offset encoded in the dual-echo [1-13 C]pyruvate images to correct geometric distortion and improve spatial registration. The proposed scheme was validated in a phantom study, and in vivo [1-13 C]pyruvate and [1-13 C]lactate rat images were acquired with intentional transmit frequency deviations to assess the dual-echo 3D EPI sequence.

RESULTS: The phantom study demonstrated improved spatial registration in off-resonance corrected images. Close agreement was observed between metabolic kidney signal and the underlying anatomy in rat imaging experiments. Relative to a single-echo acquisition, the coherent addition of the two corrected echoes provided the expected increase in signal-to-noise ratio by approximately 2.

CONCLUSION: A novel dual-echo 3D EPI acquisition sequence for integrated off-resonance correction in hyperpolarized 13 C imaging was developed and demonstrated. The proposed sequence offers clear advantages over flyback EPI for time-resolved metabolic mapping. Magn Reson Med 79:643-653, 2018. © 2017 International Society for Magnetic Resonance in Medicine.